In many applications, it is desirable to switch high voltages. One application requiring switching high voltage levels is in conjunction with bipolar electrostatic chucks that are used in the semiconductor wafer manufacturing industry. Bipolar electrostatic chucks are used to hold semiconductor wafers during the manufacturing process. To hold a wafer, direct current (DC) high voltages of opposite polarity are applied to a pair of chuck electrodes. After the wafer is lowered onto the chuck electrodes, an electrostatic charge is applied to the wafer. The charges on the wafer and the chuck electrodes produce an electrostatic attractive force that presses the wafer against the upper face of the chuck. The chucking voltages are set to a high enough value to produce an electrostatic force between the wafer and the chuck that is adequate to prevent wafer movement during subsequent process steps. Typically, the chucking voltage may range between .+-.300 and .+-.3000 volts.
After completion of any semiconductor fabrication process steps, the wafer must be "dechucked," or removed from the electrostatic chuck. To remove the wafer, the chucking voltage is typically turned off and the chuck electrodes and the wafer connected to ground. Grounding the components removes the respective charges that have accumulated on the chuck electrodes and the wafer during the chucking process. The elimination of the electrostatic force therefore allows the wafer to be easily removed.
A recognized problem with the above-described method of removing a wafer from an electrostatic chuck is that repetitively charging and grounding the chuck electrodes causes the electrodes to retain some charge. If any electrostatic attractive force remains between the wafer and the chuck due to the retained charge, excessive force may be required to remove the wafer. The force can crack the wafer or cause the wafer to pop off the chuck into a position from which it is difficult to retrieve and align properly in an automatic assembly line.
Several different techniques have been suggested to solve the wafer sticking problem. In one technique, the polarity of the chucking voltages placed on the chucking electrodes is periodically reversed. For example, after a period of operation during which the first electrode is kept at +3000 volts and the other electrode kept at -3000 volts, the polarity of each electrode is switched so that the first electrode is operated at -3000 volts and the second electrode operated at +3000 volts. Periodically changing the polarity of the chucking voltage has been found to remove the tendency of the chuck electrodes to retain an electrostatic charge.
FIG. 1 depicts a representative circuit of a switching power supply 8 that may be used to reverse the polarity of a chucking voltage applied to one of the chuck electrodes. A first high voltage DC generator 10 produces a high voltage on an output line 12, for example 3000 volts. A second high voltage DC generator 14 produces a high voltage of the opposite polarity on a load line 16, for example -3000 volts. The load line is connected to one of the chuck electrodes for application of the chucking voltage. In order to allow the polarity of the chucking voltage to be reversed, two switches are provided in the power supply. A first high voltage switch SW1 is connected between the output of the first high voltage generator 10 and the load line 16. A second high voltage switch SW2 is connected between the second high voltage generator 14 and ground. When switch SW1 is closed, switch SW2 open, and the second high voltage generator turned off, the load line 16 is maintained at +3000 volts. When switch SW1 is open, switch SW2 closed, and the second high voltage generator turned on, the load line 16 is maintained at -3000 volts. Appropriately switching the two switches and the high voltage generators therefore allows the voltage on the load line to be switched between the two high voltage potentials, in this particular case causing a reversal in the output voltage polarity on one of the chuck electrodes. A second power supply similar to that shown in FIG. 1 is required to switch the polarity of the other chuck electrode.
While the design of the switching power supply 8 is very simple, in practice it is very difficult to construct the supply and achieve suitable performance. The implementation difficulty lies in the construction of switches SW1 and SW2. Power supply manufacturers have attempted to use high voltage electromechanical relays as switches in the power supply. Two problems have arisen with the use of relays, however. First, high voltage electromechanical relays are notoriously unreliable and have a tendency to fail periodically. Especially in automated assembly lines where the power supply switches are continuously cycled on and off, such failure is generally undesirable.
Second, the switching time of electromechanical relays is uncontrolled and often too fast to allow for compensation by other components within the switching power supply. For example, the high voltage provided on output line 12 must generally be maintained at an exact level for use in the electrostatic chuck. When switch SW1 is closed very quickly, the first high voltage generator 10 is connected with the load line 16 causing the load placed on the generator to increase suddenly. Because of the rapidity of the relay switching, the high voltage generator cannot immediately compensate for the change in load, causing the output from the generator to droop. Although the first high voltage generator will ultimately compensate for the added load, the droop in the output from the power supply is detrimental to the operation of the electrostatic chuck. For example, the droop in output voltage may result in dechucking and loss of a wafer or damage to the chuck.